Once the songs for the recital have been selected, it is helpful to research the history of the songs and the life and times of the composer to fully understand the works you are performing. Some of this information will also be important for preparing the printed program and program notes. Two sources are indispensable as a beginning for almost all research in music:

The New Grove Dictionary of Music and Musicians. Stanley Sadie, ed. 2nd ed. London: Macmillan; New York: Grove's Dictionaries, 2001. 29 v. Ref ML100.N48 2001
An authoritative and comprehensive encyclopedia of Music containing extensive articles on every aspect of music. Many of the articles have selective bibliographies. The New Grove Dictionary of Opera. Stanley Sadie, ed. London: Macmillan Press; New York: Grove's Dictionaries of Music, 1992. 4 v. Ref ML102.O6 N5 1992 The New Harvard Dictionary of Music.
Don Michael Randel, ed. Cambridge: Belknap Press of Harvard University Press, 1986. Ref ML100.N485 1985
This work is the standard source for definitions of musical terms. The entries are thorough and authoritative and many include brief bibliographies.

A useful history of song literature is:

Stevens, Denis.
A History of Song. London: Hutchinson, 1960. ML2800.S8

These general bibliographies provide citations to major studies of song literature.

Marco, Guy A. Opera: A Research and Information Guide. Garland Reference Library of the Humanities, v. 468. New York: Garland, 1984. Ref ML128.04 M28 1984 Seaton, Douglass.
The Art Song: A Research and Information Guide. Music Research and Information Guides, v. 6. Garland Reference Library of the Humanities, v. 673. New York: Garland, 1987. Ref ML128.S3 S33 1987
Includes sections on song composers, poets, song texts translations, analyses, printed music, and performance practice.

More in-depth information about a composer can be found by in books by using a subject search on the composer's name. Books specifically about songs by individual composers, are located by a subject search on the composer's name followed by the uniform title, Songs:

SCHUBERT, Franz, 1797-1828. Songs

BRAHMS, Johannes, 1833-1897. Songs

The following books are a few examples:

Capell, Richard. Brahms's Lieder: An Introduction to the Songs for One and Two Voices. Translated by C. Leonard Leese. London: Oxford University Press, H. Milford, 1928. ML410.B8.F73 Moore, Gerald. Poet's Love: The Songs and Cycles of Schumann. New York: Taplinger, 1981. MT410.S4 M65 Porter, E.G. Schubert's Song Technique. London: D. Dobson, 1961. ML410.S3 P6 Reed, John. The Schubert Song Companion. New York: Universe Books, 1985.Ref ML410.S3 R265 1985 Sams, Eric. The Songs of Hugo Wolf. London: Methuen, 1961. ML410.W8 S2 Sams, Eric. The Songs of Robert Schumann. New York: W.W. Norton, 1967. ML410.S4 S3 Schumann, Robert. Dichterliebe: An Authoritative Score Historical Background, Essays in Analysis, Views and Comments. Ed. by Arthur Komar. Norton Critical Scores. New York: W.W. Norton, 1971. Mini M1621.4.S39 D54 1971 Schumann, Robert. The Singer's Schumann. Edited, with introductions and translations by Thilo Reinhard. New York, NY: Pelion Press, 1989. ML54.6.S4 R413 1989 Stark, Lucien.
A Guide to the Solo Songs of Johannes Brahms. Bloomington: Indiana University Press, 1995. MT121.B73 S73 1995